Trunked-radio systems are well-known in the prior art. Trunked-radio systems are commonly used by a number of different organizations. Often, the organizations are public-safety organizations, such as police departments, fire departments, EMS, public works, etc.
In a trunked-radio system, when a user (e.g., a police officer) wants to transmit, the user presses a Push to Talk button on their radio (referred to as “keying the radio”). Once the user keys the radio, the radio communicates with a controller over a known control channel to acquire a frequency/channel to use for that desired transmission. This process of acquiring a channel is transparent from the user's perspective.
Once the radio has acquired a channel, the radio emits a Talk Permit Tone, which audibly indicates to the user that the radio has successfully acquired a channel and that the user may begin speaking.
Radios made by different radio manufacturers generally use different Talk Permit Tones. Therefore, radios of different organizations may emit different Talk Permit Tones. A problem arises when users of one trunked-radio system (e.g., police department) want or need to communicate with users of another trunked-radio system (e.g., fire department). Such communication across multiple organizations may be desirable in many different situations, such as in emergency scenarios where police, fire, and EMS need to be in real-time communication with one another. Similarly, there may be times when other types of communications systems need to be able to communicate with users on a trunked-radio system. For example, a user of a cellular phone may want to talk to a user of a trunked-radio system.
Radio systems exist in the prior art for patching trunked-radio systems to other audio sources, such as other trunked-radio systems or cellular systems. These systems are often referred to as intercommunication systems, connectivity systems, and/or interoperability systems. An intercommunication system generally connects to one radio from each trunked-radio system being patched, as well as any other audio sources being patched (such as a cellular telephone). To patch an audio source to a trunked-radio system, an intercommunication system generally keys the radio to which it is connected (i.e., requests a channel) as soon as the intercommunication system knows that it needs to send audio to the trunked-radio system.
A problem occurs when patching other audio sources to a trunked-radio system because the users of the other audio sources do not provide any advance notice that they are about to start talking; instead, they just start talking. This can be a problem because there is an initial delay in trunked-radio systems while the transmitting radio acquires a channel. In other words, the transmitting user in one system begins speaking before the radio in the trunked-radio system to which that user is being patched has acquired a channel. As a result, the intercommunication system must acquire a channel on the trunked-radio system “on demand,” or in real-time.
Acquiring a channel in real-time can be problematic because (1) a channel is not available immediately; and (2) the intercommunication system does not know how long it will take to acquire a channel since the time to do so varies based on factors such as congestion, over-subscription, weak signals, etc. To address this problem, some prior-art intercommunication systems use a fixed transmit delay. When using transmit delay, the prior-art intercommunication system keys the radio as soon as it knows it needs to send audio to that radio's trunked-radio system, and it then begins buffering the audio until the transmit delay period expires. When the transmit delay expires, the buffered audio is released to the radio for transmission. This results in the transmission being delayed by the preset (or fixed) amount of time of the transmit delay. Prior-art intercommunication systems using transmit delay, however, suffer from three major problems: (1) latency; (2) audio clipping; and (3) outright failure.
Turning to the first problem, intercommunication systems using transmit delay introduce latency into the system that, in many situations, is unacceptable. The amount of time it takes to acquire a channel on the trunked-radio system is indeterminate (e.g., it may take a half of a second, or it may take five seconds). Because of this, the transmit-delay period is often set to the maximum amount of time it takes to acquire a channel (i.e., the worst-case scenario). Using the maximum transmit delay, however, introduces a significant amount of highly undesirable latency into the system because all communications to the trunked-radio system are delayed by the maximum amount. Therefore, in a situation where the transmit delay is set to 5 seconds but the channel is acquired in 1 second, the remaining 4 seconds of delay are wasted.
Turning to the second problem, intercommunication systems using transmit delay may unacceptably clip audio. Some prior-art connectivity systems set a short transmit delay to avoid the latency problem discussed above. When it takes longer than the transmit delay to acquire a channel, the beginning of the audio sent to the trunked-radio system will be clipped (i.e., not heard). For example, in a situation where the transmit delay is set to 2 seconds but the channel is acquired in 3 seconds, then 1 second of the audio will be clipped from the beginning. This could create a potential scenario in which a person says “Don't Shoot!” over the radio, but that audio is clipped and the person listening only hears “Shoot!”
Turning to the third problem, intercommunication systems using transmit delay may fail to acquire a channel Instead of receiving a Talk Permit Tone, the intercommunication system may receive a Failure Tone, indicating that channel acquisition has failed. In this case, the intercommunication system does not know that it does not have a channel and simply plays the audio to the trunked-radio system, but that audio will be unheard.
Although the above problems are described in the context of trunked-radio systems, the same problems apply to other audio communication systems that operate similarly to a trunked-radio system, such as push-to-talk wireless/cellular system in which the phones play out a Talk Permit Tone after a brief delay, once they have acquired a channel (or network path).
Accordingly, a need exists for devices and methods that patch audio sources into a trunked-radio system that allows for real-time communications without introducing unnecessary latency or audio clipping and that handles failures.